1. Field of the Invention
The invention relates in general to a method for manufacturing non-volatile memory and a structure thereof, and more particularly to a method for manufacturing non-volatile memory having silicon nanocrystals and a structure thereof.
2. Description of the Related Art
The non-volatile memory has the function of permanent memory. Of the semiconductor application elements, the non-volatile memory having the advantages of small volume, fast storage access speed, and low power consumption is often used in electronic products with portable mass storage such as digital camera, music player, and memory card. However, the non-volatile memory is already facing a size problem. As the size of the non-volatile memory is miniaturized, the film thickness of the tunneling oxide layer must be miniaturized accordingly (for example, being small than 5 nanometers). The tunneling oxide layer, undertaking many times of read/write access, is susceptible to defects which lead to the occurrence of leakage path and the loss of the electrical charges of the memory and make the memory incapable of maintaining its charging and memory function.
According to the current element structure of the non-volatile memory having nanocrystals, the nanocrystals exist in the thin film are used for replacing the conventional poly-Si floating gate in storing electrical charges. As deep levels formed by the nanocrystals are discrete traps, there is no interaction between the stored electrical charges, and the stored electrical charges will not lose easily due to the tunneling oxide layer being too thin or having defects. These features help to increase read and write speeds, reduce an operating voltage, and make a high density feasible.
The most difficult part in the manufacturing process of the memory element having nanocrystals is that the control of nanocrystal formation. The annealing process for the nanocrystals requires a high temperature (>900° C.) treatment needed for manufacturing silicon nanocrystals, but the high temperature treatment will damage the Si-substrate, even the glass substrate.
There are two conventional methods of manufacturing silicon nanocrystals: one is by precipitating silicon nanocrystals and the other is by growing silicon nanocrystals.
The conventional method of precipitating silicon nanocrystals is disclosed below. Firstly, a thermal silicon oxide layer having a thickness of 15 nanometer is deposited. Next, the thermal silicon oxide layer forms a Si-rich oxide layer by silicon (Si) ion implantation, wherein the Si-rich oxide layer is made from Si1.75O2, and the depth of ion implantation is 10 nm. Then, silicon nanocrystals are precipitated by a rapid thermal annealing (RTA) process at 1000° C. and 2% of oxygen content.
The conventional method of growing silicon nanocrystals is disclosed below. Firstly, an amorphous silicon (a-Si) layer grows by a process of low pressure chemical vapor deposition (LPCVD). Next, the a-Si layer grows silicon nanocrystals by high temperature furnace annealing process.
No matter the silicon nanocrystals are formed by rapid thermal annealing process or high temperature furnace annealing process, a high temperature (>1000° C.) annealing processing is required, and the high temperature heat treatment will damage the Si-substrate and the glass substrate which requires an even lower temperature. On the other hand, to obtain the Si-rich layer by ion implantation is not only time-consuming but also hard to control. As the panel size tends to become larger and larger, ion implantation will affect production capacity.
Besides, if the crystals being formed have a low density or dispersed distribution, the crystals will be incapable of storing sufficient electrical charges. If the electrons are too big or too close to each other, the electrons may jump to nearby nanocrystals or penetrate the underneath oxide layer and result in leakage of current.
The currently available technologies including ion implantation, heat treatment precipitation method or chemical vapor chromatography synthesis all have the disadvantages, such as quantum dots being too less, size being hard to control, and distribution being non-uniform, processing time being too long and operating temperature being too high, and fragile thin film. Thus, how to provide a non-volatile memory having nanocrystals with high density and uniform distribution has become a focus to the manufacturers.